Chapter 2

Connection-Oriented Networks - Harry Perros 1

Chapter 2:SONET/SDH and GFP

TOPICS– T1/E1– SONET/SDH - STS 1, STS -3 frames– SONET devices– Self-healing rings– Generic frame protocol, and Data over SONET


T1/E1

• Time division multiplexing allows a linkto be utilized simultaneously by manyusers

MUX

DEMUX

N input links

N output links

link

12

N

12

N


• The transmission is organized into frames.• Each frame contains a fixed number of time slots.• Each time slot is pre-assigned to a specific input

link. The duration of a time slot is either a bit or abyte.

• If the buffer of an input link has no data, then itsassociated time slot is transmitted empty.

• A time slot dedicated to an input link repeatscontinuously frame after frame, thus forming achannel or a trunk.


Pulse code modulation

• TDM is used in telephony• Voice analog signals are digitized at the

end office using Pulse Code Modulation.• A voice signal is sampled 8000 times/sec,

or every 125 µsec.• A 7-bit or 8-bit number is created every

125 µsec.


The Digital Signal (DS) andITU-T standard

• A North American standard that specifies how tomultiplex several voice calls onto a single link.

• The DS standard is a North American standard andit is not the same as the international hierarchystandardized by ITU-T.

• Both standards are independent of the transmission.


T carrier / E carrier• The DS signal is carried over a carrier system

known as the T carrier.– T1 carries the DS1 signal,– T2 carries the DS2 signal etc

• The ITU-T signal is carried over a carrier systemknown as the E carrier.

• The DS and ITU-T hierarchy is known as theplesiochronous digital hierarchy (PDH). (Plesionmeans “nearly the same”, and chronos means“time” in Greek).


Digital signal number Voice channels Data Rate (Mbps)DS0 1 0.064DS1 24 1.544

DS1C 48 3.152DS2 96 6.312DS3 672 44.736

DS3C 1344 91.053DS4 4032 274.176

Table 2.1: The North American Hierarchy

Level number Voice channels Data Rate (Mbps)0 1 0.0641 30 2.0482 120 8.4483 480 34.3684 1920 139.2645 7680 565.148

Table 2.2: The international (ITU-T) hierarchy


The DS1 signal

• 24 8-bit time slots/frame– Each time slot carries 8 bits/ 125 µsec, or the channel

carries a 64 Kbps voice.– Every 6th successive time slot (i.e, 6th, 12th, 18th,

24th, etc), the 8 bit is robbed and it is used forsignaling.

• F bit: Used for synchronization. It transmits thepattern: 10101010…

FTime

slot 1

Time

slot 2

Time

slot 3

Time

slot 24. . .


• T1:– Total transmission rate: 24x8+1 = 193 bits per 125 µ

sec, or 1.544 Mbps• E1

– 30 voice time slots plus 2 time slots forsynchronization and control

– Total transmission rate: 32x8 = 256 bits per 125 µsec,or 2.048 Mbps


Fractional T1/E1

• Fractional T1 or E1 allows the use of onlya fraction of the T1 or E1 capacity.

• For example: if N=2, then only two timeslots are used per frame, which correspondsto a channel with total bandwidth of 128Kbps.


Unchannelized frame signal

• The time slot boundaries are ignored by thesending and receiving equipment.

• All 192 bits are used to transport data followed bythe 193rd framing bit.

• This approach permits more flexibility intransmitting at different rates.

• This scheme is implemented using proprietarysolutions.


The synchronous optical network(SONET)

• Proposed by Bellcore (Telecordia).– It was designed to multiplex DS-n signals and

transmit them optically.• ITU-T adopted the synchronous digital

hierarchy (SDH), as the internationalstandard.– It enables the multiplexing of level 3 signals

(34.368 Mbps)


STS, STM, OC

• The electrical side of the SONET signal isknown as the synchronous transport signal(STS)

• The electrical side of the SDH is known asthe synchronous transport module (STM).

• The optical side of a SONET/SDH signal isknown as the optical carrier (OC).


The SONET/SDH hierarchyOptical

level

SONET

level

(electrical)

SDH

level

(electrical)

Data rate

(Mbps)

Overhead

rate

(Mbps)

Payload

rate

(Mbps)

OC-1 STS-1 - 51.840 1.728 50.112

OC-3 STS-3 STM-1 155.520 5.184 150.336

OC-9 STS-9 STM-3 466.560 15.552 451.008

OC-12 STS-12 STM-4 622.080 20.736 601.344

OC-18 STS-18 STM-6 933.120 31.104 902.016

OC-24 STS-24 STM-8 1244.160 41.472 1202.688

Oc-36 STS-36 STM-12 1866.240 62.208 1804.932

OC-48 STS-48 STM-16 2488.320 82.944 2405.376

OC-96 STS-96 STM-32 4976.640 165.888 4810.752

OC-192 STS-192 STM-64 9953.280 331.776 9621.504

OC-768 STS-768 STM-256 39813.120 1327.104 38486.016

OC-N STS-N STM-N/3 N*51.840 N*1.728 N*50.112


• SONET/SDH is channelized.– STS-3 consists of 3 STS-1 streams, and each STS-

1 consists of a number of DS-1 and E1signals.– STS-12 consists of 12 STS-1 streams

• Concatenated structures (OC-3c, OC-12c, etc)– The frame of the STS-3 payload is filled with

ATM cells or IP packets packed in PPP or HDLCframes.

– Concatenated SONET/SDH links are commonlyused to interconnect ATM switches and IP routers(Packets over SONET).


The STS-1 frame structure1 2 3 4 5 6 … 90

1 1 2 3 4 5 6 … 90

2 91 92 93 94 95 96 … 180

3 181 182 183 184 185 186 … 270

4 271 272 273 274 275 276 … 360

5 361 362 363 364 365 366 … 450

6 451 452 453 454 455 456 … 560

7 561 562 563 564 565 566 … 630

8 631 632 636 634 635 636 … 720

9 721 722 723 724 725 726 … 810


• Main features– The frame is presented in matrix form and it is

transmitted row by row.– Each cell in the matrix corresponds to a byte– The first three columns contain overheads– The remaining 87 columns carry the

synchronous payload envelope (SPE), whichconsists of user data, and additional overheadsreferred to as the payload overhead (POH)


An SPE may straddle betweentwo successive frames

Frame i

Frame i+1

1 2 3 4 5 6 . . . 901

2

3

4

5

6

7

8

9

276

276275

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

1

2

3

4

5

6

7

8

9


The section, line, and path overheads

Section

Line

STS-1 STS-1

A B

regeneratorregeneratorSTS-1

A1

A12

STS-12

. . .

STS-1

B1

B12STS-12

. . .

Section Section Section Section

LineLine

Path


• Section: a single link with a SONET deviceor a regenerator on either side of it.

• Line: A link between two SONET devices,which may include regenerators

• The section overhead in the SONET frameis associated with the transport of STS-1frames over a section, and the lineoverhead is associated with the transport ofSPEs over a line.


The SONET stack

Section

Line

Path

Photonic

Section

Line

Path

Photonic

Section

Line

Photonic

Section

Photonic

Section

Photonic

Section

Line

Photonic

AiA Regenerator Regenerator BiB


STS-1: Section and line overheads

SOH

LOH

Column1 2 3

1 A1 A2 J02 B1 E1 F13 D1 D2 D34 H1 H2 H35 B2 K1 K26 D4 D5 D67 D7 D8 D98 D10 D11 D129 Z1 Z2 E2


• The following are some of the bytes in thesection overhead (SOH) :– A1 and A2: These two bytes are called the

framing bytes and they are used for framealignment. They are populated with the value1111 0110 0010 1000 or 0xF628, whichuniquely identifies the beginning of an STS-frame.

– J0: This is called the section trace byte and itis used for to trace the STS-1 frame back to itsoriginating equipment.


– B1: This byte is the bit interleaved parity byteand it is commonly referred to as BIP-8. It isused to perform an even-parity check on theprevious STS-1 frame after the frame has beenscrambled. The parity is inserted in the BIP-8field of the current frame before it is scrambled

– E1: This byte provides a 64 Kbps channel canbe used for voice communications by fieldengineers.


• The following are some of the bytes in the lineoverhead (LOH) that have been defined:– H1 and H2: These two bytes are known as the pointer

bytes, and they contain a pointer that points to thebeginning of the SPE within the STS-1 frame. Thepointer gives the offset in bytes between the H1 andH2 bytes and the beginning of the SPE.

– B2: This is similar to the B1 byte in the sectionoverhead and it is used to carry the BIP-8 parity checkperformed on the line overhead section and thepayload section. That is, it is performed on the entireSTS-1 frame except the section overhead bytes.


The path overhead bytes

J1

B3C2G1F2

H4Z3

Z4Z5

J1B3C2G1F2H4 Z3Z4Z5

Location of the POH The POH bytes


• The following are some of the bytes that havebeen defined:– B3: This byte is similar to B1 used in the section

overhead and B2 used in the line overhead. It is used tocarry the BIP-8 parity check performed on the payloadsection. That is, it is performed on the entire STS-1frame except the section and line overhead bytes.

– C2: This byte is known as the path signal label and itindicates the type of user information carried in theSPE, such as, virtual tributaries (VT), asynchronousDS-3, ATM cells, HDLC-over-SONET, and PPP overSONET.


The STS-1 payload

• The payload consists of user data and thepath overhead.

• User data:– Virtual tributaries: sub-rate synchronous data

streams, such as DS-0, DS-1, E1, and entireDS-3 frames

– ATM cells and IP packets


Virtual tributaries

• The STS-1 payload is divided into sevenvirtual tributary groups (VTG).

• Each VTG consists of 108 bytes (12 columns)• Each VTG may carry a number of virtual

tributaries, i.e., sub-rate streams.


• The following virtual tributaries have beendefined:– VT1.5: This virtual tributary carries one DS-1

signal and it is contained in three columns, thattake up 27 bytes. Four VT1.5’s can betransported in a single VTG.

– VT2: This virtual tributary carries an E1 signalof 2.048 Mbps. VT2 is contained in fourcolumns, that is it takes up 36 bytes. ThreeVT2’s can be carried in a single VTG.


• VT3: This virtual tributary transports theunchannelized DS-1 signal. A VT3 iscontained in 6 columns that takes up 54 bytes.This means that a VTG can carry two VT3s.

• VT6: This virtual tributary transports a DS-2signal, which carries 96 voice channels. VT6 iscontained in 12 columns, that is it takes up 108bytes. A VTG can carry exactly one VT2.


ATM cells

• Mapped directly onto the SPE. An ATMcells may straddle two SPEs.

10Cell 1 Cell 2

Cell 2 Cell 3

Cell 14 Cell 15

Cell 15

9041

9

2

8

3

POH


IP packet over SONET

• IP packets are first encapsulated in HDLC andthe resulting frames are mapped into the SPEpayload row by row as in the case above forATM cels.

10 9041

9

2

8

3POH

7E 7E 7E

7E7E7E


• IP packets can also be encapsulated in PPPinstead of HDLC.

• A frame may straddle over two adjacent SPEs, asin the case of ATM.

• The interframe fill 7E is used to maintain acontinuous bit tstream


The STS-3 frame structure

Overhead section Payload section

1 2 3 4 5 6 7 8 9 10 11 12270

. . .

1st S

TS-1

1st S

TS-1

1st S

TS-1

1st S

TS-1

1st S

TS-1

2nd S

TS-1

2nd S

TS-1

2nd S

TS-1

2nd S

TS-1

2nd S

TS-1

3rd S

TS-1

3rd S

TS-1

3rd S

TS-1

3rd S

TS-1

3rd S

TS-1


• The channelized STS-3 frame is constructed bymultiplexing byte-wise three channelized STS-1frames. As a result:– Byte 1, 4, 7, … , 268 of the STS-3 frame contains byte

1, 2, 3, … , 90 of the first STS-1 frame.– Byte 2, 5, 8, …, 269 of the STS-3 frame contains byte

1, 2, 3, … , 90 of the second STS-1 frame– Byte 3, 6, 9, …, 270 of the STS-3 frame contains byte

1, 2, 3, … , 90 of the third STS-1 frame.• This byte-wise multiplexing, causes the columns

of the three STS-1 frames to be interleaved in theSTS-3 frame


• The first 9 columns of the STS-3 framecontain the overhead part and theremaining columns contain the payloadpart.

• Error checking and some overhead bytesare for the entire STS-3 frame, and they areonly meaningful in the overhead bytes ofthe first STS-1 frame.


SONET/SDH devices

• Several different equipment exist:– Terminal multiplexer (TM)– Add/drop multiplexer (ADM)– Digital cross connect (DCS)


• It multiplexes a number of DS-n or E1 signalsinto a single OC-N signal

• It consists of a controller, low-speed interfacesfor DS-n or E1 signals, an OC-N interface, and atime slot interchanger (TSI)

• It works also as a demultiplexer

. . .

DS-n

OC-N

DS-n

TM

The terminal multiplexer (TM):


• It is a more complex version of the TM• It receives an OC-N signal from which it can

demultiplex and terminate (i.e., drop) anynumber of DS-n or OC-M signals, where M<N,while at the same time it can add new DS-n andOC-M signals into the OC-N signal.

. . .

DS-n. OC-M

OC-N OC-NADM

The add/drop multiplexer (ADM)


SONET ringsADM

1ADM

2

ADM3

ADM4

OC3

OC3

OC3

OC3

• SONET/SDH ADM devices are typically connected toform a SONET/SDH ring.

• SONET/SDH rings are self-healing, that is they canautomatically recover from link failures.


An example of a connection

A

B

TM1

TM2

ADM1

ADM2

ADM3

ADM4

DS1

OC12

DS1

OC12

OC12

OC12

OC3

OC3


• A transmits a DS-1 signal to TM 1• TM 1 transmits an OC-3 signal to ADM 1• ADM 1 adds the OC-3 signal into the STS-

12 payload and transmits it out to the nextADM.

• At ADM 3, the DS-1 signal belonging to Ais dropped from the payload andtransmitted with other signals to TM 2.

• TM 2 in turn, demultiplexes the signals andtransmits A’s DS-1 signal to B.


• Connection setup:– Using network management procedures the

SONET network is provisioned appropriately.This is an example of a permanent connection.

– It remains up for a long time.• The connection is dedicated to user A

whether the user transmits or not.


A digital cross connect (DCS)

Ring 1 Ring 2ADM

ADM

ADM

ADM

ADM

ADM

DCS

• It is used to interconnect multiple SONET rings• It is connected to multiple incoming and outgoing OC-N

interfaces. It can drop and add any number of DSn and/orOC-M signals, and it can switch DSn and/or OC-Msignals from an incoming interface to any outgoing one.


Self-healing SONET/SDH rings

• SONET/SDH rings have been speciallyarchitected so that they are available 99.999% ofthe time (6 minutes per year!)

• Causes for ring failures:– Fiber link failure due to accidental cuts, and

transmitter/receiver failure– SONET/SDH device failure (rare)


Automatic protection switching (APS)

• SONET/SDH rings are self-healing, that is, thering’s services can be automatically restoredfollowing a link failure or degradation in thenetwork signal.

• This is done using the automatic protectionswitching (APS) protocol. The time to restore theservices has to be less than 50 msec.


Protection schemes: point-to-point

• Schemes for link protection– dedicated 1+1– 1:1– Shared 1:N

ADM

Working

ProtectionADM


Working/protection fibers

• The working and protection fibers have tobe diversely routed. That is, the two fibersuse separate conduits and different physicalroutes.

• Often, for economic reasons, the two fibersuse different conduits, but they use thesame physical path. In this case, we saythat they are structurally diverse.


Classification of self-healing rings

• Various ring architectures have beendeveloped based on the following threefeatures:– Number of fibers

• 2 or 4 fibers– Direction of transmission:

• Unidirectional bidirectional– Line or path switching


Number of fibers: 2- or 4-fiber rings

Two-fiber ring: fibers 1, 2, 3, and 4 areused to form the working ring (clockwise),and fibers 5, 6, 7, and 8 are used to formthe protection ring (counter-clockwise).

1

2

3

4

5

6

7

8

ADM 1 ADM 2

ADM 3ADM 4

ADM 1 ADM 2

ADM 3ADM 4


• In another variation of the two-fiber ring, each set of fibersform a ring which can be both a working and a protectionring. In this case, the capacity of each fiber is divided intotwo equal parts, one for working traffic and the other forprotection traffic.

• In a four-fiber SONET/SDH ring there are two workingrings and two protection rings, one per working ring.

1

2

3

4

5

6

7

8

ADM 1 ADM 2

ADM 3ADM 4

ADM 1 ADM 2

ADM 3ADM 4


Direction of transmission

• Unidirectional ring:– signals are only transmitted in one

direction of the ring.• Bidirectional ring:

– signals are transmitted in both directions.


Line and path switching

• Path switching: Restores the traffic on thepaths affected by a link failure (a path is anend-to-end connection between the pointwhere the SPE originates and the point whereit terminates.)

• Line switching: Restores all the traffic thatpasses through a failed link.


Based on these three features, we have thefollowing 2-fiber or 4-fiber possible ringarchitectures:– Unidirectional Line Switched Ring (ULSR)– Bidirectional Line Switched Ring (BLSR)– Unidirectional Path Switched Ring (UPSR)– Bidirectional Path Switched Ring (BPSR)


Of these rings the following three areused:– Two-fiber unidirectional path switched ring

(2F-UPSR)– Two-fiber bidirectional line switched ring

(2F-BLSR)– Four-fiber bidirectional line switched ring

(4F-BLSR)


Two-fiber unidirectionalpath switched ring (2F-UPSR)

ADM 1 ADM 2

ADM 3ADM 4

5

264 8

3

7

A

Protection ring

Working ring

1B


• Features:– Working ring consists of fibers 1, 2, 3 and 4,

and the protection ring of fibers 5, 6, 7, and 8.– Unidirectional transmission means that traffic

is transmitted in the same direction. Atransmits to B over fiber 1 of the working ring,and B transmits over fibers 2, 3, and 4 of theworking ring.

– Used as a metro edge ring to interconnectPBXs and access networks to a metro core ring


• Self-healing mechanism:– Path level protection using the 1+1 scheme. The

signal transmitted by A is split into two. Onecopy is transmitted over the working fiber 1, andthe other copy is transmitted over the protectionfibers 8, 7, and 6.

– During normal operation, B receives twoidentical signals from A, and selects the onewith the best quality. If fiber 1 fails, B willcontinue to receive A’s signal over theprotection path. The same applies if there is anode failure.


Two-fiber bidirectional line switchedring (2F-BLSR)

ADM 1 ADM 2 ADM 3

ADM 4

7

396 12

5

11

A B1

8

4

2

10

ADM 5ADM 6

C


• Features:– Used in metro core rings.– Fibers 1, 2, 3, 4, 5, and 6 form a ring, call it ring 1, on

which transmission is clockwise. Fibers 7, 8, 9, 10, 11,and 12 form another ring, call it ring 2, on whichtransmission is counter-clockwise.

– Both rings 1 and 2 carry working and protection traffic.This is done by dividing the capacity of each fiber onring 1 and 2 to two parts. One part is used to carryworking traffic and the other protection traffic.

– A transmits to B over the working part of fibers 1 and2 of ring 1, and B transmits to A over the working partof fibers 8 and 7 of ring 2.


• Self-healing mechanism:– The ring provides line switching. If fiber 2 fails

then the traffic that goes over fiber 2 will beautomatically switched to the protection part ofring 2.

– That is, all the traffic will be re-routed to ADM3 over the protection part of ring 2 using fibers7, 12, 11, 10, and 9. From there, the traffic foreach connection will continue on following theoriginal path of the connection.


Four-fiber bidirectional line switchedring (4F-BLSR)

Working rings

ADM 1 ADM 2 ADM 3

ADM 4

A B

ADM 5ADM 6

Protection rings


• Features– Two working rings and two protection rings.

The two working rings transmit in oppositedirections, and each is protected by aprotection ring which transmits in the samedirection.

– The advantage of this four-fiber ring is that itcan suffer multiple failures and still function.In view of this, it is deployed by long-distancetelephone companies in regional and nationalrings.


• Self-healing operation (span switching):– If a working fiber fails, the working traffic will

be transferred over its protection ring. This isknown as span switching.

ADM 1 ADM 2 ADM 3 ADM 1 ADM 2 ADM 3

Normal operation Span switching


• Self-healing operation (ring switching):– Often, the working and protection fibers are

part of the same bundle of fibers. When thebundle is cut the traffic will be switched to theprotection fibers. This is known as ringswitching.

B

ADM 1 ADM 2 ADM 3

ADM 4

A

ADM 5ADM 6

Working

Protection

ADM 1 ADM 2 ADM 3

ADM 4

A

B

ADM 5ADM 6

Working

Protection

B


Generic Framing Procedure (GFP)

• This is a light-weight adaptation schemethat permits the transmission of differenttypes of traffic over SONET/SDH and inthe future, over G.709.


• GFP permits the transport ofa) frame-oriented traffic, such as Ethernet, andb) block-coded data for delay-sensitive storage

area networks (SAN) transported by networkssuch as Fiber Channel, FICON, and ESCON

over SONET/SDH and G.709.• GFP is a result of joint standardization

effort by ANSI committee T1X1.5 and ITU-T.

• It is described in ITU-T recommendationG.7041


Privatelines Ethernet ESCON FICON Fiber

Channel

Frame Relay POS

ATM

SONET/SDH

WDM/OTN

GFP

Voice Data (IP, MPLS, IPX) SAN

DM

Video

Existing and GFP-based transport options for end-user applications

HDLC


The GFP stack

GFP

GFP client-dependent aspects

GFP client-independent aspects

SONET/SDH G.709

Ethernet IP over PPP SAN data


GFP frame structure

Payload

Core header

Payload lengthPayload length

Core HECCore HEC

Payload header

Payload

Payload FCS

• GFP core header– Payload length indicator

(PLI) - 2 bytes. It gives thesize of the payload.

– Core HEC (cHEC) - 2bytes. It protects the PLIfield. Standard CRC-16enables single bit errorcorrection and multiple biterror detection.


The GFP payload structure

Payload header

Payload

Payload FCS

Payload type

Payload type

Type HEC

Type HEC

0-60 bytesof

extension header

Payload FCS

Payload FCS

Payload FCS

Payload FCS

PTI

UPI

PFI EXI


GFP payload headervariable-length area from 4 to 64 bytes.

• Payload type - 2 bytes– Payload type identifier (PTI) - 3 bits.

Identifies the type of frame:• User data frames , Client mgmt frames

– Payload FCS indicator (PFI) - 1 bit.Identifies if there is a payload FCS

– Extension header identifier (EXI) - 4 bits.Identifies the type of extension header.

– User payload identifier (UPI) - 8 bits.Identifies the type of payload

• Frame-mapped Ethernet• Frame-mapped PPP (IP, MPLS)• Transparent-mapped Fiber Channel• Transparent-mapped FICON• Transparent-mapped ESCON• Transparent-mapped GbE

• Type HEC (tHEC) - 2 bytes. It protects thepayload header. Standard CRC-16.

Payload type

Payload type

Type HEC

Type HEC

0-60 bytesOf

Extension header

PTI

UPI

PFI EXI


GFP payload trailer

Payload header

Payload

Payload FCS

Payload FCS

Payload FCS

Payload FCS

Payload FCS

• Optional 4-byte FCS.– CRC-32– Protects the contents of

the payloadinformation field.


GFP-client independent functions

• The client independent sublayer supportsthe following functions:– Frame delineation– Client/frame multiplexing– Payload scrambler– Client managment


Frame delineation

• The framedelineationmechanism is similarto the one used inATM.

• The cHEC is used toassure correct frameboundaryidentification

hunt

Presync

Sync

CorrectcHEC

2ndcHEC match

Non-correctablecore header error

No 2ndcHEC


• Operation:– Under normal conditions, the GFP receiver

operates in the Sync state. The receiverexamines the PLI field, validates the cHEC,and extracts the framed higher-level PD. Itthen moves on to the next GFP header.

– When an uncorrectable error in the coreheader occurs (i.e., cHEC fails and more thanone bit error is detected), the receiver entersthe Hunt state.


• Hunt state:– Using the cHEC it attempts to locate the

beginning of the next GFP PDU, moving onebit at a time (Same as in ATM - see Perros “Anintroduction to ATM networks, Wiley 2001.

– Once this is achieved it moves to the Pre-Syncstate, where it verifies the beginning of theboundary of the next N GFP PDUs.

– If successful, it moves to the Sync state,otherwise it moves back to the hunt state.


Frame multiplexing

• Client data frames and client managementframes are multiplexed, with client dataframes having priority over clientmanagement frames.

• Idle frames are inserted to maintain acontinuous bit flow (rate coupling)


GFP client-specific functions

• The client data can be carried in GFPframes using on of the two adaptationmodes:– Frame-mapped GFP (GFP-F) applicable to

most packet data types– Transparent-mapped GFP (GFP-T) applicable

to 8B/10B coded signals


Frame-mapped GFP

• Variable length frames such as:– Ethernet MAC frames,– PPP/IP packets– HDLC-framed PDUs

can be carried in the GFP payload.• One frame per GFP payload.• Max. size: 65,535 bytes


Transparent-mapped GFP

• Fiber Channel, ESCON, FICON, GigabitEthernet high-speed LANs use 8B/10Bblock-coding to transport client data andcontrol information.

• Rather than transporting data on a frame-by-frame basis, the GFP transparent-mappedmode, transports data as a stream ofcharacters.


• Specifically, the individual characters arede-mapped from their client 8B/10B blockcodes and then mapped into periodic fixed-length GFP frames using 64B/65B blockcoding.

• This reduces the 25% overhead introducedby the 8B/10B block-coding.

• Also, transparent mapping reduces latency,which is important for storage relatedapplications


• The first step, is to decode the 8B/10Bcodes. The 10 bit code is decoded into itsoriginal data or control codeword value.

• The decoded characters are then mappedinto 64B/65B codes. A bit in the 65-bitcode indicates whether the 65-bit blockcontains only data or control characters arealso included

• 8 consecutive 65-bit blocks are groupedtogether into a single superblock.

• A GFP frame contains N such superblocks.


Data over SONET/SDH (DoS)

• The DoS architecture provides an efficientmechanism to transport data coming frominterfaces such as: Ethernet, Fiber Channel,ESCON/FICON over SONET/SDH.

• It relies on a combination of– GFP,– Virtual concatenation, and– Link capacity adjustment scheme (LCAS)


Virtual concatenation• This procedure maps an incoming traffic stream

into a number of individual sub-rate payloads.• The sub-rate payloads are switched through the

SONET/SDH network independently of eachother

• At the destination, they are used to reconstruct theoriginal traffic stream.


Example• Let us consider the case of transporting the

1 GbE signal over SONET/SDH.• According to the specifications, an STS-

48c (2,488 Gbps) has to be used, thusleaving a lot of unused capacity.

• Using the virtual concatenation scheme, 7independent STS-3c (7x155,520 = 1,088)can be employed to carry the 1 GbE signalat full rate.


This works as follows:• At the transmitter the incoming stream is

de-multiplexed and distributed in somefashion over 7 different payloads, each anSTS-3c.

• Intermediate SONET/SDH nodes only seedifferent payloads and they are not awareof the concatenation

• At the destination, the seven flows getmultiplexed into the single original GbEstream.


Link capacity adjustment scheme(LCAS)

• This scheme permits to dynamically adjustthe number of sub-rate payloads allocatedto a traffic stream, whose transmission ratemay vary over time.

• LCAS can be also used when re-routingtraffic due to a failure.

Date post:	27-Oct-2014
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Chapter 2

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